Titan

This near-infrared, color mosaic from NASA's Cassini spacecraft shows the sun glinting off of Titan's north polar seas. Credit: NASA/JPL-Caltech/University of Arizona/University of Idaho

Discovery

Titan was discovered on Mar. 27, 1655 by the Dutch astronomer Christiaan Huygens.

Overview

Saturn's largest moon Titan is the second largest moon in our solar system, second only to Jupiter's Ganymede, which is only 2 percent larger. With a mean radius of 1,600 miles (2,575 km), Titan is bigger than Earth's moon, and even larger than the planet Mercury.

Titan is the only moon in our solar system that has clouds and a dense atmosphere, mostly nitrogen and methane. It is also the only other place in the solar system known to have an earthlike cycle of liquids flowing across its surface.

Orbit

Titan orbits Saturn at a distance of about 759,000 miles (1.2 million km), taking 15 days and 22 hours to complete a full orbit. Titan is tidally locked in synchronous rotation with Saturn, and permanently presents one face to the planet as it completes its orbit.

Surface

Cassini has revealed that Titan's surface is shaped by rivers and lakes of liquid ethane and methane (the main component of natural gas). These liquids form clouds from which the liquid gases sometimes rain from the sky as water does on Earth.

Ligeia Mare, shown here in a false-color image from NASA's Cassini mission, is the second largest known body of liquid on Saturn's moon Titan. It is filled with liquid hydrocarbons, such as ethane and methane, and is one of the many seas and lakes that bejewel Titan's north polar region. Credit: NASA/JPL-Caltech/ASI/Cornell

On Titan it is so cold (-290 degrees Fahrenheit or -179 degrees Celsius) that water plays the role of rock and lava, and flowing methane carves river channels and fills great lakes with liquid natural gas. Vast regions of tall dunes stretch across the landscape -- dunes whose "sand" is composed of dark hydrocarbon grains. The dunes are not unlike those seen in the desert of Namibia in Africa. Volcanism may occur as well, but with liquid water as the lava.

Titan has few impact craters, meaning that its surface must be relatively young and some combination of processes erases evidence of impacts. This is the case for Earth as well; craters on our planet are eroded by the relentless forces of flowing liquid (water, in Earth's case) and wind. These forces are present on Titan as well. Tectonic forces -- the movement of the ground due to pressures from beneath -- also appear to be at work on Saturn's largest moon.

Atmosphere

Titan is the only moon in the solar system with a thick atmosphere. Titan's atmospheric pressure is about 60 percent greater than Earth's -- roughly the same pressure found at the bottom of a swimming pool. Because Titan is less massive than Earth, its gravity doesn't hold onto its gaseous envelope as tightly, so the atmosphere extends to an altitude 10 times higher than Earth's - nearly 370 miles (600 km) into space.

Titan's atmosphere is mostly nitrogen (about 95 percent) and methane (about 5 percent), with small amounts of other carbon-rich compounds. High in the atmosphere, methane and nitrogen molecules are split apart by the sun's ultraviolet light and high-energy particles accelerated by Saturn's magnetic field; the products of this splitting recombine to form a variety of organic molecules. (Organic molecules contain carbon and hydrogen, and often include nitrogen, oxygen and other elements important to life on Earth.)

Some of the compounds produced by the splitting and recycling of methane and nitrogen in the upper atmosphere create a kind of smog - a thick, orange-colored haze that obscures the moon's surface from view. And some of the heavy, carbon-rich compounds also fall to the surface. Some of these hydrocarbons go on to form grains that make up the "sand" of vast dune fields on Titan's surface.

One of Titan's great mysteries is the source of its methane, which makes all of this complex chemistry possible. Since sunlight breaks down methane in the atmosphere, there must be a source that replenishes what is lost. Researchers suspect methane could be belched into Titan's atmosphere by cryovolcanism, or volcanoes with water as lava.

Exploration

In 1994, NASA's Hubble Space Telescope recorded pictures of Titan, which suggested that a huge bright continent exists on the hemisphere that faces forward in orbit. These Hubble results didn't prove that liquid seas existed, however; only that Titan has large bright and dark regions on its surface.

Titan's surface remained shrouded in secrecy below the clouds until July 2004. That's when NASA's Cassini spacecraft arrived. Cassini was specially designed to peer through Titan's haze with radar and in certain colors of light, called spectral windows, that allow a glimpse of what lies below. During dozens of flybys, the Cassini orbiter has mapped a large fraction of Titan's surface and made detailed studies of its atmosphere. Cassini also carried the European-built Huygens probe, which parachuted through Titan's atmosphere in 2005 to make the first landing on a body in the outer solar system.

How Titan Got its Name

The name Titan comes from a generic term for the children of Ouranos (Uranus) and Gaia in ancient Greek mythology. In the stories, the Titans were the ancestors of the human race. The Titans were known to have devoured the limbs of Dionysus, the son of Zeus. Enraged, Zeus struck the Titans with lightning. (Zeus had intended this child to have dominion over the world.) The lightning burned the Titans to ashes, and from the ashes, mankind was formed.